What body surface area is
Body surface area (BSA) is the total external area of your skin, expressed in square metres. Nobody measures it directly outside of a research lab — the original determinations involved coating subjects in paper or plaster and measuring the wrapping. Everything in clinical use today is an estimate from height and weight, fitted to those early direct measurements.
It matters because a surprising amount of physiology scales with area rather than mass. Heat loss happens across a surface. So does gas exchange, and — usefully for medicine — so does the maximum dose of a cytotoxic drug a body will tolerate. When a number needs to be normalised for “body size” in a way that weight alone gets wrong, BSA is the standard denominator.
The four formulas
Mosteller (1987)
The simplest of the set and the most widely used in hospitals, because it needs one square root and agrees with Du Bois to within a couple of percent across the normal adult range. If a chemotherapy order sheet shows a BSA, it was probably this.
Du Bois & Du Bois (1916)
The original power-law fit, derived from just nine directly measured subjects — one of whom was a child. It remains the reference equation that later formulas are validated against, despite its tiny sample, and it tends to read slightly low in obese and very small patients.
Haycock (1978)
Fitted to a sample spanning infants through adults, which is why it is the common choice in paediatrics. Note the higher weight exponent — it leans more on mass and less on height than Du Bois does.
Gehan & George (1970)
A least-squares fit to 401 directly measured surface areas, the largest validation set of the four. Statistically the best-supported equation of the group; it never displaced Mosteller in practice because it is harder to do in your head.
Where they disagree
| Body | Mosteller | Du Bois | Haycock | Gehan-George |
|---|---|---|---|---|
| 160 cm, 55 kg | 1.56 | 1.56 | 1.57 | 1.58 |
| 178 cm, 75 kg | 1.93 | 1.93 | 1.93 | 1.93 |
| 190 cm, 120 kg | 2.52 | 2.47 | 2.55 | 2.53 |
| 100 cm, 16 kg (child) | 0.67 | 0.66 | 0.67 | 0.68 |
In the middle of the adult range the four are effectively interchangeable. The gap opens up at high body weights, where Du Bois reads lowest — a systematic difference that is large enough to change a rounded drug dose, which is why institutions standardise on one formula rather than letting each clinician pick.
Why medicine doses by BSA
Three main uses:
- Cytotoxic chemotherapy. Doses are prescribed in mg/m². The convention dates to mid-century animal work showing that maximum tolerated dose scaled with surface area across species better than with body weight. Because the therapeutic window is narrow, even a modest scaling improvement was worth adopting — and once trial protocols were written in mg/m², the unit was locked in.
- Cardiac index. Cardiac output divided by BSA, in L/min/m². Normal is roughly 2.5–4.0. Indexing is what lets you compare a 55 kg patient's heart to a 110 kg patient's.
- Renal function and other clearances. Estimated GFR is reported per 1.73 m², the historical average adult BSA — see the GFR calculator for that normalisation in action.
Worth knowing: BSA dosing is openly criticised. It ignores body composition, organ function and enzyme activity, all of which affect drug clearance more than surface area does. It survives because it is standardised, cheap, and better than dosing by weight alone — not because anyone thinks skin area is the mechanism.
BSA vs BMI
These get confused because both combine height and weight. They are not alternatives:
| BSA | BMI | |
|---|---|---|
| Question | How big is this body, physiologically? | Is this weight high for this height? |
| Direction of height | Taller → larger BSA | Taller → lower BMI at the same weight |
| Used for | Drug dosing, cardiac index, eGFR normalisation | Weight classification, population screening |
| Has healthy range | No | Yes (18.5–25) |
There is no such thing as a “good” BSA. A large BSA is not a risk factor; it is a bigger denominator. If you want the weight-status question, use the BMI calculator; if you want the composition question, the body fat calculator and lean body mass calculator are the right tools. BSA answers a scaling question, and only that.